This invention relates to a roll for a foil-drawing calender or the like of the type which is substantially solid and has a central longitudinal bore hole, is supported in journals at its ends and has associated devices for compensating deflection caused by the line pressure.
Known rolls of this type have an outside diameter of about 700 mm and a diameter of the central longitudinal bore hole of about 200 mm. If the roll has been manufactured by centrifuging, this longitudinal bore hole depends on the fabrication. In the finished roll it can be used to conduct a fluid heating or cooling medium through the roll.
The term "substantially solid" indicates the considerable remaining wall thickness of the roll body of about 250 mm. The wall thickness is so large that the loss of bending resistance moment due to the longitudinal bore hole as compared with a completely solid roll is not more than 10%.
The practically complete solidness is an integral feature of a roll intended for a foil-drawing calender or similar applications, since such a roll must be capable of calibration, i.e., it must be capable, due to its dimensional stiffness, of equalizing local differences in the thickness or the compressibility of the plastic compound offered and discharging from the roll gap a foil which has a thickness as constant as possible over the width of its web.
The line pressures required in the rolling of plastic foil are very considerable. Thus, line pressures on the order of magnitude of 3700 N/cm are required for rolling low-pressure PVC (polyvinylchloride) and even 6300 N/cm for rolling high-pressure PVC. In spite of the quasi-solidity of the rolls and in spite of the relatively large diameter with the customary working widths of 2 m, such line pressures already lead to deflections of the roll as a whole, which, without special measures, would lead to unpermissible tolerances of the foil thickness at the edges and in the center of the web.
A further integral feature of the known foil-drawing calender is therefore the use of devices for compensating this deflection caused by the line pressure. These devices are very expensive because of the dimensional stiffness of the rolls and the occurrence of high line pressures. Three different measures are taken side by side throughout, namely, what is called roll bending, i.e., the introduction of bending forces which counteract the deflection caused by the line pressure; a bombage, i.e., slight diameter differences along the roll produced by grinding the shape of the roll accordingly; and an oblique adjustment of rolls relative to each other, so that the roll axes of an interacting pair of rolls do not lie in the same plane but the one roll is, in the form of a very steep screw, so to speak, placed around the other roll. The design and structural expense which is necessary to accommodate all three measures in one and the same machine, is obvious.
In spite of this considerable expense, it is not possible today at that to run a large range of line pressures with one and the same machine. Differently designed machines are required for processing softer plastics and for processing harder plastics.
It is an object of the present invention to provide a simpler compensating system for the deflections caused by the line pressure, usable over a larger range of line pressures while retaining the solidity or dimensional stiffness of the roll.